Sensitizing and hardening photographic emulsions



atented Dec. 13, 1938 2,139,774 SENSITIZING AND HARDENING PHOTO- PATENTOFFICE GRAPHIC EMULSIONS Samuel E. Sheppard, Robert C. Houck, andRaymond H. Lambert, Rochester, N. Y., assignors,

by mesne assignments,

pany, Jersey City, N. J.,

Jersey to Eastman Kodak Coma corporation of New i No Drawing.Application July 10, 1936 Serial No. 90,000

4 Claims.

This invention relates to finishing photographic emulsions andparticularly, to sensitizing and hardening gelatino-silver halideemulsions.

In practice it has been found necessary to treat 5 photographic gelatinor emulsion solutions or layers with a hardening agent in order toincrease the resistance of the gelatin or emulsion layer to swelling orsoftening, particularly when processed at increased temperatures.Compounds which have been used for this purpose are chrome alum,formaldehyde, and other reagents. In application Serial No. 745,754,filed September 27, 1934 in thenames of S. E. Sheppard and R. C. Houck,there is disclosed the use of hydroxy aldehydes up to and including thepentoses for this purpose.

As is well known, a gelatino-silver halide emulsion when first mixed isquite slow or insensitive, and unsatisfactory for making fast negativeemulsions. The emulsion mixture is, therefore, digested or ripened inorderto increase the sensitivity. This ripening is ordinarilyaccomplished by the action of heat or by digestion with ammonia, eitheralone or in combination with heating. After treatment in this way theemulsion may be jelled and set and then re-melted and coated on theemulsion support. The hardening agent is added at some period subsequentto the digestion, before coating the emulsion on the support.

In many cases the degree of hardening produced in this way, as indicatedby the melting point, swelling, behavior on photographic processing andother ways, is not complete after drying the product, but-a greater orlesser amount of so-called after hardening occurs on storage 5 of theplates or films. This may continue through a period of months'and mayequal or surpass in degree the initial hardening on first drying. Thisafter hardening, therefore, gives rise to a product whose properties arevariable with time unless an inconvenientiy long period of storage priorto use is involved. One consequence of this is generally reduction ofspeed of development, but other objectionable changes in photographicproperties may ensue. A further objection is that it is necessary torepeatedly subject samples of the plates or films to incubation tests inorder to ascertain the amount of hardening that has taken place.'

50 It is, therefore, an object of the present invention to provide ameans for eliminating after hardening and for producing a more stableemulsion which will show less change in regard to sensitometric andphysical properties, such as hardening and swelling, on storage. Afurther object is to provide a novel method for hardening photographicemulsions. A still further object is to provide a photographic emulsionwhich will be resistant to relatively high temperatures withoutobjectionable fogging or de-sensitizing. Other objects will appearhereinafter.

We have discovered that with a considerable number of hardening agentsthe slow process of after hardening may be eliminated and the emulsionbrought to a stable condition by heating the coated, dried plates orfilms, the emulsion of which has been subjected to little or no wetdigestion, for a specified time (period of hours only) at specifictemperatures and relative humidities. i

Our process of heat accelerated hardening would have only limitedapplication as carried out on emulsions after ordinary finishing, thatis, emulsions which had been subjected to digestion or ripening. Sinceour process is equivalent to a period of incubation under tropicalconditions, photographic emulsions prepared in the usual way andsubjected to our process would probably, if not certainly, show rapiddeterioration, such as fogging, hypersensitization or desensitization,decrease in contrast, etc., on storage.

On the other hand, it is possible to treat a photographic emulsion tomoist incubation at specified temperature and relative humidities,according to our invention, and thereby, to produce an emulsion which isfairly stable over a long period of time and which does not exhibit atendency. to after hardening on storage. This treatment increases thesensitivity of the emulsion, and avoids the necessity of the usual wetdigestion.

In accordance with our process a gelatino-silver halide emulsionprepared in the usual way by precipitating a silver halide in thepresence of gelatin had added thereto a hardening agent and it is thencoated ontoa suitable support at the usual temperature, for example 40C. and dried in the usual way. The plate or film is then subjected tothe heat treatment described below and the combined sensitizing andhardening action performed in this way.

This dry digestion or moist incubation has a number of advantages overthe usual wet digestion in which the emulsions are ripened prior toaddition of the hardening agent, in that the progress of the hardeningis more conveniently controlled and that there is less tendency toincreased fogging with increasing sensitization. This is shown by thefollowing tables indicating the speed of plates, subjected to digestionand obtained by moist incubation (for 3.5 hours) was nearly, twice thatobtained by wet digestion (at twenty minutes) for a corresponding amountof fog formation.

emulsions at a specified temperature and relative humidity containing.various hardening agents, where the emulsion was not subjected todigestion prior to adding the hardening agent. In each casethe timerequired to produce a certain melting point has been comparedwith theproper time that would be required to store the emulsion at roomtemperature in order toreach a melting point of 80 C. to 90 C.

pH6.2-6.7 (except where otherwise; noted).

70 C.60% relative humidity.

P 'ri me oft f ercentea ing a o me of incubation at room Hardening agentgfi ggf gi 3 2252? M l ing 1 1 11 tempere ture to reach gelatin tive hu-80 midity Hours 0. 0 45 Formaldehyde 10 2 months. 0: 0 e2 0.18 1.0 903-4 days. 2. 0 97 0. 0 34 1. 0 34 1 '8 it 0 Months. Arabmose- 4' o 40 5.5 51 6. 0 64 0.0 sees Z 0 8 22 Months. 3. 0 89 0. 0 e4 Questionable.Several DIBCBtYLL'. 4044 anontbs, perhaps a yea -s1 Acetonyl acetone 4-2g 4 weeks 23. 0 45-55 0. 0 35-82 Acetonyl acetone 0 3:8 2 34 weeks.

23.0 70 0. 0 55 Acetoxime pH 10.0 aged 10 mos 6.0 m 0 90+ }Quesfionable.

(Very slowly) 'Ierephthalic aldehyde 6- 0 32 Chloral pH 8.0 aged 11 mos14. 0 2:8 3? (Slowly melting) Methylal pH 8.0 aged 17 mos 14. 0 8:3 1}Years. Hexamethylene tetramine 1. 0 8:8

a. o 2-8 }e-12 months (estimated). 6.0 g g }e-12 months (estimated).

Wet digestion at 60 C.

Fog 4 min- Time, Speed, H. minutes and D. 3255 Moist incubation at 60 C.

. Fog 4 min- Tune, Speed, H.

utes develophours and D. men;

The following examples illustrate the increased 7 hardening achieved byheating gelatin or gelatin By melting point is meant the temperature atwhich the emulsion or gelatin dissolves in water on heating the water.

In the above examples we have referred to the use of a pI-l of 6.2 to6.7 with a temperature of 70 C. and relative humidity of 60%. Theselimits are for the purposes of example only and-do not exhaust thepossibilities of our invention. It

. is known that during digestion of a photographic emulsion thesensitivity varies with the pH but there are practical pH limits above.which it is not desirable to go. In a similar manner the temperature ofour treatment may be greater or less than 70 C. and may vary from about40 C. to

90 C. The relative humidity may also vary somewhat within the range ofabout 50% to 80%, depending upon the amount and nature of the hardeningagent used.

Where the emulsion is subjected to moist incubation without the usualwet digestion, before or after coating it on the support, thetemperature of treatment is preferably 60 C. to 70 0., and the relativehumidity therefor between 50% and 80%.

Efiect of heat on hardening of gelatin emulsion by various hardeningagents 10 Although we have disclosed the use of a number of hardeningagents together with a preferred method of carrying out our invention itis to be understood that we are not limited to the compounds and methodsof procedure described herein, but that our invention is limited only bythe scope of the appended claims.

What we claim is:

1. The process of hardening gelatino silver halide emulsions andsubstantially eliminating after-hardening, which comprises adding ahardening agent to the emulsion, coating the emulsion on a support anddrying it, and subjecting the emulsion to moist incubation at a relativehumidity of between 50% and 80% and a temperature of between 40 C. and90 C. conjointly with ripening, for such length of time as will bringthe emulsion to substantially uniform hardness.

2. The process of sensitizing and. hardening photographic emulsionswhich comprises precipitating a silver halide in the presence of gelatinto form an emulsion, adding a hardening agent to the emulsion, coatingthe emulsion on a support, drying and then subjecting the emulsion tomoist incubation at a relative humidity between about and about 80%, anda. temperature of about C. to C. for such length of time as will bringthe emulsion to substantially uniform hardness.

3. The process of sensitizing and hardening photographic emulsions whichcomprises precipitating a silver halide in the presence of gelatin toform an emulsion, adding a hardening agent to the emulsion, coating theemulsion on a support, drying and then subjecting the emulsion toincubation in air at about 60% relative humidity and 60 C. to 70 C. forsuch length of time as will bring the emulsion to substantially uniformhardness.

4. The process of sensitizing and hardening photographic emulsions whichcomprises precipiq tating a silver halidein the presence of gelatin toform an emulsion, adding a hardening agent to the emulsion, coating theemulsion on a support, drying and then subjecting the emulsion toincubation in air at 50% to relative humidity and 60 C. to 70 C. forsuch length of time as will bring'the emulsion to substantially uniformhardness. 4

SAMUEL E. SHEPPARD. ROBERT C. HOUCK. RAYMOND H. LAMBERT.

